Efficient combustion furnaces for heating, ventilating and air conditioning (HVAC) equipment applications are typically provided with a motor driven so-called ventilating or inducer blower which draws air through the combustion passageways of the furnace heat exchanger to improve the efficiency of the combustion and heat transfer processes and to prolong the life of the furnace. Selection of the proper speeds of the inducer blower drive motor for multistage combustion furnaces, in particular, has been a somewhat nettlesome problem. Combustion furnaces which include electronic controls have been developed wherein the inducer blower motor speed is controlled based on opening and closing of pressure switches which measure the pressures developed by the inducer blower at one or more particular points in the air flowpath.
Moreover, so-called learning algorithms have been developed which require setting a blower default speed for multistage furnaces for the low firing rate and high firing rate which is the first speed that the inducer motor will be controlled to when a call for the low firing rate or high firing rate is signaled to the furnace controller. The inducer blower then “learns” a speed based on opening and closing of the pressure switches. Still further, typically, a low speed limit is defined in the control system program to avoid the combustion gas control valve closing prematurely. U.S. Pat. Nos. 6,257,870 and 6,377,426 to Hugghins, et al. and assigned to the assignee of the present invention disclose and claim methods for setting inducer blower operating speeds.
However, for multistage furnaces including, for example, three stage furnaces, it is desirable to maintain the inducer flow pressures above a predetermined setpoint which is particularly important at low firing rates to avoid low combustion gas pressures which could create undesirable combustion characteristics. This occurs because the gas valve output pressure tracks the inducer or system pressures in the aforementioned type of furnace. Still further, it is desirable to simplify the “learning” of the inducer blower motor speeds for respective furnace firing rates in multistage furnaces, including three stage furnaces. In accordance with the present invention, improvements in determining and setting inducer blower speeds and operating pressures have been realized and attendant advantages enjoyed as a result.